1. Field of the Invention
The present invention relates to a thin-film transistor to be used for fabricating a liquid crystal display or the like and, more particularly, to a thin-film transistor that reduces the off-current.
2. Description of the Related Art
Referring to FIG. 5 showing an equivalent circuit of an active matrix liquid crystal display employing thin-film transistors as switching elements, a plurality of parallel scanning electrode lines G1, G2, . . . and Gn, which will be inclusively designated by G, and a plurality of parallel signal electrode lines S1, S2, . . . and Sm, which will be inclusively designated by S, are extended across each other, the scanning electrode lines G are connected to a scanning circuit 1, and the signal electrode lines S are connected to a signal supply circuit 2. Thin-film transistors (switching elements) 3 are formed near the intersection points of the scanning electrode lines G and the signal electrode lines S, respectively. A capacitor 4 and a liquid crystal element 5 are connected in parallel to the drain of each thin-film transistor 3.
In the circuit shown in FIG. 5, the scanning electrode lines G1, G2, . . . and Gn are scanned sequentially to turn on the thin-film transistors 3 on each scanning electrode line G simultaneously, and the signal supply circuit 2 charges through the signal electrode lines S1, S2, . . . and Sm the capacitors 4 corresponding to the liquid crystal elements 5 to be driven, among the capacitors 4 connected to the thin-film transistors 3 in the on-state. The stored signal charges keep exciting the corresponding liquid crystal elements 5 after the thin-film transistors 3 have been turned off until the next scanning cycle is started. Consequently, the liquid crystal elements 5 are controlled by control signals to display picture elements. Thus, the liquid crystal elements are driven statically even though the external driving circuits, i.e., the scanning circuit 1 and the signal supply circuit 2, drive the liquid crystal elements 5 in a time sharing mode.
FIGS. 3 and 6 show a portion of the conventional active matrix liquid crystal display illustrated by the equivalent circuit of FIG. 5. Referring to FIGS. 5 and 6, the scanning electrode lines G and the signal electrode lines S are formed so as to cross each other on a transparent substrate 14, such as a glass substrate, and the thin-film transistor 3 is formed between the scanning electrode line G and the signal electrode line S.
As shown in FIGS. 3 and 6, the thin-film transistor 3 comprises a gate electrode 8 formed by extending a portion of the scanning electrode line G, an insulating layer 16 covering the gate electrode 8, a semiconductor layer 20 of amorphous silicon (a-Si) formed on the insulating layer 16, and a drain electrode 10 and a source electrode 12 formed opposite to each other with a gap therebetween by processing a conductive layer, such as an aluminum layer, formed on the semiconductor layer 20. An ohmic contact layer 22 is formed by doping the upper surface of the semiconductor layer 20. The thin-film transistor 3 shown in FIG. 3 is generally called a thin-film transistor of a channel etch type. The laminated structure thus fabricated is covered with a protective film 18. The drain electrode 10 is connected through a contact hole 24 formed in the protective film 18 to a pixel electrode 15, and the source electrode 12 is connected to the signal electrode line S. A portion of the semiconductor layer 20 corresponding to the gap between the drain electrode 10 and the source electrode 12 is a channel region 26. An orientation film, not shown, is formed on the pixel electrode formed on the protective film 18, a transparent substrate provided with an orientation film is put on the orientation film formed over the pixel electrode, and a liquid crystal is sealed in the space between the orientation films to complete the active matrix liquid crystal display.
An electric field created by the pixel electrode 15 is applied to the molecules of the liquid crystal to control the orientation of the molecules of the liquid crystal. When the transparent substrate 14 is illuminated by light rays emitted by a backlight from behind, the direction of polarization of light rays changes according to the orientation of the liquid crystal, some liquid crystal elements transmits light rays and other liquid crystal elements intercept light rays to display desired pictures.
Since the electrodes and the like that does not transmit light are extended in portions of the liquid crystal display corresponding to the thin-film transistors 3, light rays emitted by the backlight from behind the substrate 14 do not contribute to displaying pictures. However, those light rays fall on the lower surfaces of the thin-film transistors 3. When the backlight is disposed behind the substrate 14 of the liquid crystal display, part of light rays emitted by the backlight and transmitted by the substrate 14 falls on the semiconductor layer 20 as indicated by the arrows A in FIG. 3, and part of the light rays falling on the semiconductor layer 20 is reflected by the inclined portions 28 of the lower surface of the semiconductor layer 20. Consequently, the conductivity of the illuminated semiconductor layer 20 increases and a photocurrent flows. Therefore, when the thin-film transistor 3 is being driven, an undesirable leakage current flows though the circuit is kept in the off-state by the gate electrode 8. The leakage current increases the off-current when driving the liquid crystal, adversely affecting the light transmission characteristic of the liquid crystal display.
If electric charge of a fixed polarity is applied continuously to the liquid crystal, the dc component biases the ions of the orientation film in contact with the liquid crystal, and adsorbed charge creates an electric field which causes sticking. Since the light transmission characteristic of the liquid crystal remains unchanged even if the polarity of the voltage applied to the pixel electrode 15 is reversed, the liquid crystal is driven by an ac voltage to prevent sticking.
However, when the liquid crystal is driven by an ac voltage, a parasitic capacitance is created, and the gate voltage is applied to the pixel electrode 15 and causes the dynamic voltage shift of the potential of the pixel electrode 15. The parasitic capacitance that cause the voltage shift is created because the insulating layer 16 formed in part of the active matrix liquid crystal display functions as a capacitor.
A parasitic capacitance is created in the structure shown in FIG. 3 by the insulating layer 16 formed between the gate electrode 8 and the drain electrode 10. Since a large portion of the drain electrode 10 overlaps the gate electrode 8, the gate-drain capacitance C.sub.GD is comparatively large, which increases the parasitic capacitance. The gate-drain capacitance C.sub.GD generates switching noise when the liquid crystal is driven. Research and development activities have been made to form the gate electrode 8 in the least possible size so that the portions of the drain electrode 10 and the source electrode 12 overlapping the gate electrode 8 are reduced to suppress the gate-drain capacitance C.sub.GD to the least possible extent.
Another problem is the conventional liquid crystal display is that a leakage current flows from the interface between the semiconductor layer 20 and the gate insulating layer 16 to the drain electrode 10 and the source electrode 12, and the off-current increases when the gate voltage is negative. Increase in the off-current reduces the retention and, consequently, the contrast is reduced and power is consumed unnecessarily.